1. Field of the Invention
The present invention relates to a filter element highly resistant to solvents and comprising a bundle of hollow fibers which has at least one end sealed with a thermoplastic resin material, and relates also to a method for the production thereof.
2. Description of the Prior Art
In conventional filter elements of this kind, at least one end of a bundle of hollow fibers is sealed with urethane adhesive. Use of such urethane adhesive leads to good workability when providing the at least one end of the bundle with a sealed portion. A filter module having such a conventional filter element contained in a housing exhibits sufficient durability to water. However, it is inferior in durability to an aqueous alcohol solution or organic solvents such as alcohol.
In view of the above, filter elements each comprising a bundle of hollow fibers which has at least one end sealed with a thermoplastic resin material resistant to solvents have recently been developed. Such filter elements are disclosed, for example, in Japanese Patent Non-examined Publications No. 1-218605, No. 3-245826 and No. 4-63117.
However, use of such thermoplastic resin has disadvantages in (1) that the step of forming a sealed portion requires a treating temperature higher than in the case of using urethane adhesive in order to obtain fluidity of the resin, resulting in thermal deformation of the hollow fibers and possible destruction of the inside diameters of the hollow fibers depending on a combination with the material of which the hollow fibers are made, (2) that since thermoplastic resin has viscosity higher than that of urethane adhesive, it fails to propagate over the entire spaces between the adjacent hollow fibers even if it should gain fluidity, resulting in an incomplete sealed portion, and (3) that thermoplastic resin encounters volume shrinkage larger than that of urethane adhesive during its setting, resulting in an incomplete sealed portion having crazing and/or cavities.
The first cited Japanese Publication discloses a method for the formation of a sealed portion of a bundle of hollow fibers by filling the inside of the ends of the hollow fibers with calcium carbonate so as not to induce thermal deformation, then sealing the spaces between the adjacent ends of the hollow fibers with fine particles of thermoplastic resin same as the thermoplastic resin of which the hollow fibers are made, and melt bonding the ends of the hollow fibers. This method requires the steps of removing the calcium carbonate by dissolving it in concentrated hydrochloric acid which will constitute waste matter and disposing of the waste matter, and encounters some difficulty in melt bonding the ends of the hollow fibers liquid-tightly over the entire area using finely particulate sealing material and failure to melt bond the center portion of the bundle of hollow fibers. Thus, this prior art cannot eliminate the disadvantages (2) and (3) mentioned above.
The second cited Japanese Publication discloses a method for the formation of a sealed portion of a bundle of hollow fibers, which adopts the step of causing the sealed portion to set from the lower part thereof to eliminate the aforesaid disadvantage (3) and uses, as a material for hollow fibers, engineering plastic generally exhibiting high resistance to heat, including polysulfone, polyether sulfone and polyimide and, as a sealing material, thermoplastic resin including polyolefin, polyvinyl chloride, nylon and polyester. This method requires that the melting point of the material for hollow fibers be much higher than that of the sealing material and cannot eliminate the aforesaid disadvantage (2).
The third cited Japanese Publication discloses a method for the formation of a sealed portion of a bundle of hollow fibers, which uses a first polyolefin as a material for hollow fibers and, as a sealing material, a second polyolefin having a melting point lower than that of the first polyolefin and adopts the use of a suspension of the second polyolefin for sealing the ends of the hollow fibers. The products obtained in accordance with this method, however, are not stable in liquid tightness. Thus, this method produces no satisfactory results.
The present invention has been proposed for the purpose of solving the problems mentioned above.
An object of this invention is to provide a filter element highly resistant to solvents and chemicals.
Another object of this invention is to provide a method for the production of the filter element at low cost.